On Nov. 28, Tiago Mendes broke his leg in a 1-0 win over Espanyol, and Atlético Madrid supporters everywhere held their breath. Here was the 34-year-old Portuguese, one of Simeone's stalwarts in the center of the park and playing arguably the best football of his career, writhing on the Vicente Calderón pitch. In response to Tiago's injury, Augusto Fernández was purchased from Celta Vigo, and even though he's done well when fit, the Argentine is a different kind of player. As it turns out, Tiago's true replacement was already in the team.
Saúl Ñíguez returned from his loan spell with Rayo Vallecano in summer 2014 and the academy graduate impressed in spurts the following season, particularly when he scored this goal against Real Madrid. Entering the 2015-16 season, Saúl was expected to continue his development as deputy to Gabi/Tiago in central midfield, but he was ushered into action when Tiago went down at the end of November.
March has just begun, and after scoring his third La Liga goal of the season in Tuesday's win over Real Sociedad, it is safe to say that Saúl has arrived a year early. The 21-year-old has stepped up and more-than-capably filled the massive void left by Tiago. He understands his duties in defense and executes them wherever in midfield he is asked to play - left, center or right. There are also games where his attacking mindset reveals itself; the lad's scored seven goals in all comps this season, making him Atlético's second-leading goalscorer behind Antoine Griezmann.
Saúl's progress might lead one to ponder what Tiago's role in the team will be once he returns to full fitness (which may or may not be this season). Saúl can't really be dropped; he's been in great form for almost three months and putting him on the bench so Tiago can test out his leg in the business end of the season seems like it would be a bad idea. Simeone won't drop Gabi, Koke or Yannick Carrasco, either. Plus, Augusto, Matías Kranevitter and Thomas Partey are all available and have contributed or are expected to contribute in the future. There are too many younger options than Tiago, and the team has purred along without him.
It is unlikely that many of these players will leave the club, either. Saúl expressed his desire to sign a lifetime contract recently and it would be suicidal to sell him at just 21 years old. Thomas could head out on loan again this summer, but otherwise, everyone else should stay put. Long story short: Tiago will remain very important at the Calderón, just probably not on the pitch. His injury was very unfortunate for him, but it gave Saúl a chance to shine, and that should not and will not be taken away from him.
My educated guess, based on how Tiago is viewed around the club, is that he will be offered a one-or two-year contract extension at good money, perhaps with the chance to score a coaching role upon his retirement. His days as a mainstay in the XI will unfortunately come to an end due to the injury, but with the future already identified and flourishing, a team-first player like Tiago can still be happy.